1. Field of the Invention
This invention relates to a rust-preventive composition for the surfaces of metals such as steel sheets used in automobiles, construction material products and so forth.
2. Description of the Prior Art
Steel sheets used in automobiles and construction material products, in particular, zinc-plated (galvanized) or zinc-alloy-plated steel sheets are coated before their use. They are passed through various steps before coating, and also are left uncoated for a fairly long time in that course. Hence, there are problems that the plated steel sheets may rust in that course, or various substances may become adsorbed on or adhere to their surfaces, so that coating materials to be applied thereafter may have a poor adhesion.
Accordingly, the plated steel sheets are subjected to chromating as primary rust-preventive treatment made before coating. Corrosion resistance attributable to this chromating may commonly only last for about 48 hours in a salt spray test made according to JIS Z-2371, and is insufficient if the chromating only is made, when left in a severe corrosive environment over a long period of time. Under such circumstances, a coating type special method of chromating is proposed in which a chromating liquid to which a silica sol has been added is used, at the stage of chromating. This method can provide a corrosion-resistant effect lasting for 100 to 200 hours in the salt spray test.
However, hexavalent chromium is contained in coating films themselves which are formed by such special chromating. This hexavalent chromium is known to cause an allergy (chromate ulcer) when the skin is stained with it. A possibility of carcinogenesis is also pointed out. Thus, there is a problem on safety. As a method not making use of any hexavalent chromium, it has been proposed to apply trivalent chromium plating further on zinc alloy plating. This method, however, can not achieve a sufficient corrosion resistance.